In Mesh et al, U.S. Pat. No. 3,814,934, commonly owned with the present invention, there is disclosed a shaft encoder including a disc driven by a shaft, the rotary motion of which is indicated by counting pulses. The rotary disc includes relatively narrow transparent slots or regions, spaced from each other by relatively wide opaque regions. Optical energy from a pair of point sources is directed through the transparent regions to be incident on a pair of optical detectors aligned with the sources. The sources, detectors, slots and regions are arranged so that the center lines between adjacent detector-source positions is less than one-half of the separation between adjacent slots, in such a manner as to provide pulses equal in number to the rotation of the shaft driving the disc, and to compensate for angular hysteresis of the shaft to at least 2.degree..
In normal operation, during most of the time while the disc is rotating, the opaque regions of the disc cause shadows to be cast on both of the detectors simultaneously. During a relatively brief interval, light is transmitted from one of the sources through a slot to the detector aligned with one of the sources. Circuitry responsive to pulses generated by the detectors being illuminated counts the generated pulses to provide an indication of extent of shaft rotation with compensation for the hysteresis effect.
It has been found that there is a tendency for the relatively narrow slots of the prior art encoder transducer to become clogged with dirt and other debris. Such clogging prevents light from being incident on the detectors when the slots pass between the detectors and sources, with obvious detrimental effects on the accuracy of the shaft rotation indication.
It has also been found that certain components in the encoder have a tendency to fail. Light emitting diodes, which form the point light sources, have a tendency to become open circuited and short circuited, in which case no optical energy is derived from them. Phototransistors, which form the light detectors, have a tendency to be become open and short circuited. An open circuit of the phototransistors has the same overall effect as an open or short circuit to the light emitting diodes, while a short circuited phototransistor produces the same effect as a constantly illuminated phototransistor. In addition, driver transistors responsive to the phototransistors have a tendency to become open and short circuited. All of these effects prevent proper derivation of an accurate indication of the extent of shaft rotation because the number of generated pulses is usually one-half of the correct number. In the previous system there is no way for the machine to indicate that any of these malfunctions have occurred. It is also somewhat difficult for an operator to realize that the number of derived pulses is only one-half of what it should be.
In the prior art, there is a relatively high likelihood of phototransistor malfunction due to a phototransistor being open circuited because relatively large voltages are normally maintained across the phototransistors during most of the time power is applied to the unit. The phototransistors have large back bias voltages across the emitter collector paths thereof most of the time because both phototransistors have shadows incident on them for about 90% of the operating time of the unit. When the phototransistors are responsive to light propagating through the slots from the light emitting diodes, relatively large voltage changes occur across them. These malfunctions are a natural consequence of the nature of the prior art encoder, wherein the phototransistors are normally maintained in a shadow, non-conducting, high impedence state. The prior art encoder has also been found to have a relatively slow response time, to limit the maximum speed of a drive shaft which is to be monitored by the encoder.
It is, accordingly, an object of the present invention to provide a new and improved apparatus for deriving a number of pulses indicative of the movement of a mechanical member that drives an encoding transducer.
It is another object of the present invention to provide a new and improved optical position encoder having higher reliability and higher frequency response than prior art encoders.
Another object of the invention is to provide a new and improved mechanical position encoder having a fail-safe mode of operation.
An additional object of the invention is to provide a new and improved optical shaft encoder wherein the encoder is prevented from operating in the event of a malfunction to a light source, light detector and circuitry responsive to the detector.
A further object of the invention is to provide an optical shaft encoder having hysteresis correction and opaque and transparent regions arranged so that the tendency for clogging of the transparent regions is substantially prevented.
A further object of the invention is to provide a new and improved optical shaft encoder having hysteresis correction and fail-safe features in the event of a failure of light sources, light detectors, and circuitry responsive to the light detectors.